1. Field of the Invention
This invention relates to a process for ion nitriding of aluminum or an aluminum alloy and an apparatus therefor.
2. Description of the Prior Art
The surface treatment of aluminum or an aluminum alloy has been limited when compared with that of an iron group material. Practically speaking, only a method for producing a film coating of alumina by anodic oxidation treatment has been employed for this purpose. This alumina coating film, which is an anodic oxidation coating film of aluminum, has a Vickers hardness of about 200 to 600 depending upon treating conditions thereof, but the wear resistance is not necessarily sufficient for many uses.
There has recently been a demand for ion nitriding of aluminum or an aluminum alloy to provide a coating film having a hardness higher than the alumina coating film.
Ion nitriding of a ferrous alloy, generally has been carried out in such a manner that a gas inside a sealed container is replaced by a nitriding gas, and a glow discharge is operated between an anode, provided at the inner wall of the sealed container or independently provided therein, and the article to be treated which is used as the cathode to generate a nitrogen ion. It has been proposed to nitride the surface of aluminum or an aluminum alloy according to this technique.
Actually, however, when the article to be treated is aluminum or an aluminum alloy the surface thereof is oxidized so that it has not been possible to obtain a nitrided coating film of aluminum or an aluminum alloy having uniformity as well as durability. In this case, an impurity, such as alumina, residing on the surface of the article to be treated is physically removed therefrom before treatment by utilizing an ion sputtering phenomenon to purify the surface thereof. Such a cleaning has been conducted in the prior art many times.
The aforesaid cleaning method is effective to remove an oxide produced on the article before treatment, but it is not possible to prevent the article from being oxidized due to an extremely small amount of oxygen or oxidizing gas mixed with the nitriding gas during the treatment.